The United States and its international space station partners have agreed to use Russian spacecraft to exchange station crews as long as U.S. space shuttles remain grounded because of the Columbia accident. The size of the station crew will decrease.

The U.S. space agency, NASA, says the three-man U.S.-Russian station crew will return to Earth in April aboard the Soyuz emergency escape craft based at the outpost.

They had originally been due to return aboard the shuttle Atlantis in March, but NASA grounded its shuttle fleet while investigators probe the cause of Columbia's disintegration on February 1.

NASA Administrator Sean O'Keefe told a U.S. congressional committee that the three crewmembers will be replaced by only two - a U.S. astronaut and Russian cosmonaut. They will arrive on another Soyuz previously scheduled as a replacement escape vehicle.

Mr. O'Keefe says the smaller crew would ease the demand on the limited supply capacity of Russian Progress rockets, which do not hold as much cargo as the shuttle. He also says the schedule of Progress visits is being accelerated.

Under the agreement reached among the 16 partner nations, the two-person rotation will occur every six months for at least a year-and-a-half during regularly planned Soyuz swaps. "We are all in agreement on the approach on how we will proceed from this stage in order to assure that we can operationally continue this important laboratory condition," he said.

The previous head of NASA, Daniel Goldin, had said that three crewmembers are the minimum needed to run the station. But Mr. O'Keefe assured lawmakers that two would be sufficient, allowing even minimal scientific research.

Under tough questioning, he vowed that the crew's safety is paramount and said it would abandon the station aboard a Soyuz craft if a Progress rocket could not meet its supply schedule during the shuttle launch moratorium. "Yes, there is a very thin margin of activity here, but there is a very important option, which is to remove the crew as quickly as possible, and there is the capacity to do that," said Sean O'Keefe.

Mr. O'Keefe says the station could remain unoccupied if necessary, but vacancy would limit NASA's ability to correct technical problems that might arise.

Because of the increased reliance on Russian spacecraft to service the station, a member of the House committee has introduced legislation to let NASA pay Moscow to build additional Soyuz and Progress spacecraft.

The measure would amend a U.S. law that bars such aid to Russia unless the Bush administration confirms that Moscow has not provided Iran with missile or weapons technology in the previous year.